Stop going to porn sites.
Never had this before, but I use Noscript and Adblock, so maybe it has something to do with that?
What web browser are you using?
The new Firefox. Noscript is pretty messed up on it (until they make a proper update), and it forgets choices I've made every time I restart the browser/computer, so it's not very convenient having it activated either.
Certainly better than getting redirected to suspicious sites though.